Civil Engineering Reference
In-Depth Information
14.3.18 Remedial works to the alignment
Where alignment errors cannot be corrected by steering, remedial works are necessary.
It is sometimes possible to create one or more cast-in-situ joints in a span to correct
its alignment. The joint is usually 100 mm thick, and is made with small aggregate
concrete. It is of course necessary to mobilise some special falsework to hold the adjacent
segments rigidly in place. If the prestressing cables are internal, it is also necessary to
fi nd a way of connecting the ducts across the joint. These joints will increase the length
of the precast span, and it may be possible to take this out in planned cast-in-situ
stitches, such as at mid-span for a deck erected in balanced cantilever.
In some decks that consist of a long run of counter-cast segments, such as long-span
cable-stayed decks, it has been considered prudent by some designers to include the
provision for one or two such cast-in-situ joints which allow errors to the alignment to
be corrected. If all goes well, they may be eliminated, widening slightly the cast-in-situ
mid-span closure.
One remedial measure sometimes proposed is to replace the 100 mm concrete joint
by a poured or injected resin joint, centimetres rather than millimetres, thick. This
is not recommended, as the resin has a much lower Young's modulus than that of
concrete, and creeps signifi cantly when subject to sustained compression or shear.
When it is not possible to adopt cast-in-situ stitches to correct the alignment, it is
necessary to recast segments. This generally involves setting up the two outer segments
of a run of three, and then casting a new segment between them. This is a diffi cult
procedure, needing very careful surveying and execution, and is very much a last
resort.
